Can dementia flip on and off like a light switch?

My 82-year-old mom has had subtle memory loss for many years, but her primary care declared she didn't have dementia after a brief exam several years ago, so she thinks she is fine and dandy. Honestly, sometimes I don't know if she is just mentally ill, has dementia or both. But she is definitely not normal. Dad died in early September, and starting the day he died, I have seen some disturbing behavior from her. She switches between her usual personality to this person who is unfamiliar, and it feels "dark". I call her "The other mother" when she becomes like this. The expression on her face becomes blank and expressionless but dark at the same time. She is more negative than normal, and she is manipulative and dishonest. On our way home from hospice the day my father died, she smirked and said "The house is all mine now. I can do whatever I want." It was very traumatizing to me. My entire life she has seemed to have OCD. It takes over 30 minutes to leave the house. She has to check lights, the oven, the curling iron, ect over and over again. Go to the toilet multiple times. When she loads the dishwasher, she will rearrange the dishes for hours on end. That behavior has always existed, and my parents used to fight about it. She creates scenarios that aren't real but once she has voiced them, she seems to believe that it is truth. For example: when at the checkout counter in the doctor's office booking the next appointment, she will wonder out loud if my brother can take her instead of me and state that she feels guilty about all the time off I am taking. She then begins creating a scenario, out loud, in which she assumes he might or might not be busy, what he will be doing, what the weather will be like and so forth. Within 5 minutes this one-sided conversation with herself will have become reality and she will proceed to plan her activities around it. She will book the appointment based on the activities she just imagined he would need to plan around and expect him to show up on that day of, without ever having talked to him. Another example, at lunch yesterday she noted that our waiter was not dressed in drag like he was when we were there at Christmas. And she again had a one-sided conversation, creating a scenario in which the waiter was reprimanded by the management and told to be more conservative if he wanted to remain employed. She ended her monologue with stating "Good thing he listened and decided to clean up his act."She will remain like this, creating her own alternate reality sometimes for days and then suddenly be herself again. She can be very sweet and pleasant when she isn't "The Other Mother". Is this how dementia acts? Or does it sound like something else?I saw a YouTube video called "A walk with Alzheimer's" and it was VERY VERY familiar. Just as the main character in the video becomes frightened of a puddle because she thinks it is a hole in the ground, my mother will react similarly at times when there is no real threat. There is one particular stretch of road that we drive, and every time we approach a specific bridge she starts gasping and shouting "Watch out!!! Be careful!!! ", but this bridge is nothing out of the ordinary and we drive over many others just like it. It isn't large, it has low steel guardrails, it isn't raised in the air, and it simply runs across a culvert between two pastures. I don't know what it is about that stretch of road that triggers her. The speed limit is only 35 mph in that area and there is usually no other traffic. It completely baffles me and her behavior is unnerving. She thinks there is nothing wrong with her. I am not sure how to handle this.The biggest question I have is in regard to the sudden change in her expression and sudden behavior change that comes over her without warning. Do dementia and/or Alzheimer's patients do this?